The question is incomplete as the options are not given. There are a lot of factors that contributed to it but major factors which led to the fall of Rome were:
The city was raided again and again by outside forces which destroyed it.
It was divided into two part during the rise of eastern empire. Both parts drifted apart with time, and failed to work together.
Economical trouble mainly due to constants wars, overspending, inflation and poor taxation system.
